In an acoustic panel of the Helmholtz resonator type, the panel has a solid hard backing plate, a series of chambers, usually of honeycomb material, and a porous face sheet. The porosity of the face sheet is selected according to the desired sound dampening required. The porous face sheet may be effected in production by the process of building up the face sheet, by the method of adhering the face sheet to the chambers, and by painting. In use the porous areas may become filled with particles. It is known to check the porosity of the face sheet by blowing air at a constant rate through the face sheet while measuring the pressure drop between the incident and the exhausting air. Using this method, the flow resistance of the face sheet is determined from the particle velocity and the pressure drop. Unfortunately, this method may not be used to test a completed Helmholtz panel. The panel may only be tested without the solid backing plate. Thus, the face sheet must either be tested in an intermediate step in production before the backing sheet or plate is in place or a coupon must be prepared along with a production part. The backing sheet is then removed from the coupon before determining the porosity as found by measuring the flow resistance of the face sheet. A completed panel may only be tested by the destruction method of removing the backing plate.
It was found that a completed acoustic panel, including a panel that has been used, may be tested with the use of a special puck or head.